Method for forming LSI pattern

ABSTRACT

First, multiple circuit patterns, which will eventually make an LSI, are designed on a cell-by-cell basis, and an initial placement is made for the circuit patterns designed. Next, optical proximity corrections are performed on at least two of the circuit patterns that have been initially placed to be adjacent to or cross each other, thereby forming optical proximity corrected patterns out of the adjacent or crossing circuit patterns. Then, it is determined whether or not optical proximity corrections can be performed effectively using the corrected patterns. If the effectiveness of the corrections is negated, a design rule defining the circuit patterns is changed to make the corrections effective. Thereafter, the initially placed circuit patterns are placed again in accordance with the design rule changed.

BACKGROUND OF THE INVENTION

The present invention relates to a method for planning a layout for anLSI pattern with optical proximity corrections ensured, a method forgenerating mask data and a method for forming an LSI pattern using thesemethods.

Recently, as a semiconductor large-scale integrated circuit (LSI) devicehas been downsized, a photolithographic step, which is one of the mainprocess steps of an LSI fabrication process, has been affected byoptical proximity effects more and more seriously. Specifically, adeviation of a feature size of a pattern actually transferred onto aresist from that of a mask pattern formed on a reticle has almostreached a non-negligible level. That is to say, if the feature size ofan originally designed pattern is automatically applied to the maskpattern, then the size of the actually transferred pattern is likely todeviate from the originally designed one. This problem is particularlynoticeable in a transistor, which plays a key role in determining theoperability of an LSI. In this specification, a feature size of apattern originally designed will be called a “designed size”, that of apattern formed on a reticle or mask a “mask size” and that of a patternactually transferred onto a resist an “actual size”, respectively.

Furthermore, every time an LSI of a new generation alternates with oneof an older generation, the feature size of the LSI should be reduceddiscontinuously. For example, process technology of 0.18 μm generationhas lately alternated with that of 0.25 μm generation. In this manner, atransistor feature size like the gate length thereof is usually reducedby about 70%. Whenever the gate length is reduced in this manner, oneexpects that the area of a cell implementing a circuit with the samefunction can also be reduced by about 50%, which is the square of 70%.This shrinkage rate is achievable by introducing a state-of-the-artexposure system using a radiation source with an even shorter wavelengthor by improving the lithographic process itself.

These days, however, it has become more and more difficult to attainthis shrinkage rate just by introducing a new system or improving theprocess. This is because the increase in deviation of an actual sizefrom a mask size often prevents design rules, which are defined toensure proper circuit operation, from achieving the 70% shrinkage rate.Examples of the design rules include the size of an extension of a gateand a contact margin.

FIG. 20( a) illustrates an originally designed pattern 100A and anactually transferred pattern 100B of an ordinary field effect transistor(FET). As shown in FIG. 20( a), the designed pattern 100A includes agate pattern 101 to be shaped into a gate layer and an active layerpattern 102 to be shaped into an active layer. In the actuallytransferred pattern 100B, the width of the gate pattern 111 is smallerthan its originally designed size, and both edge portions 111 a of thegate pattern 111 no longer exist. If the portions of the gate pattern111 overlapping with the active layer pattern 112 have been partiallylost this way, then the transistor cannot operate normally.

To solve such a problem, extensions 101 a are provided for both edges ofthe gate pattern 101 so as to extend from the active layer pattern 102in the gate width direction as shown in FIG. 20( b). As the size of aline pattern, which is called a “gate length 101 b” decreases, the areasof the edge portions of the gate pattern 101 to be lost increase. Inother words, the size 101 c of the extensions 111 a does not decreaseproportionally to the gate length 101 b. Accordingly, when the gatelength 101 b is to be reduced, the size 101 c of the extensions 101 a ofthe gate pattern 101 should be increased to ensure proper operation forthe transistor. Thus, it has become more and more difficult for thedesign rule concerning the extension size 101 c to achieve the 70%shrinkage rate.

In spite of the circumstances such as these, the design rule is stilldefined based on the deviation of an actual size from a mask size. Forexample, the design rule is defined by the 70% shrinkage rate comparedto the previous generation. Thus, to reduce the total area of thecircuit patterns, the design rule defined based on the 70% shrinkagerate is prioritized and applied to even a pattern that cannot satisfythe design rule completely, e.g., the extension size 101 c of the gatepattern 101.

Thereafter, a cell library is made up of a plurality of circuit patternsthat have been designed in accordance with the design rule. LSI chipdata is generated using the cell library to determine final conditionsfor the fabrication process. Based on these final process conditions,the deviation of an actual size from a mask size, which has been causeddue to proximity effects, is estimated, thereby generating data for amask layout that has been modified to eliminate the deviation of theactual size from the designed one. In this case, the actual sizes areestimated relative to the mask sizes using empirical models that reflectvarious conditions for estimating the actual sizes under predefinedprocess conditions.

For example, in a portion of a circuit pattern where an actual size isthinner than its mask size, the mask size is thickened compared to theoriginally designed one. Conversely, in a portion of a circuit patternwhere an actual size is thicker than its mask size, the mask size isthinned compared to the originally designed one. A mask pattern that isformed in view of optical proximity effects in this manner is called an“optical proximity corrected (OPC)” pattern.

According to the prior art method for generating LSI mask data, however,it is not until all the circuit patterns have been defined (i.e., whilemask pattern data is being generated) that the OPC patterns are made.Thus, the OPC patterns could not be formed in some cases.

For example, consider a case shown in FIG. 20( a) where the edges of thegate pattern 101 disappear. In such a situation, even if the mask sizefor the extensions 101 a of the gate pattern 101 should be corrected tomatch its actual size with the size originally designed for the circuitpattern, the size 101 c of the extensions 101 a could not be changed.For instance, this size change is impermissible if the space between theextensions 101 a and surrounding patterns thereof is of the requiredminimum size defined by the resolution limit.

Furthermore, the prior art method for generating mask data has variousdrawbacks including the following:

(1) A design rule defined without taking proximity effect correctionsinto account would result in an excessively increased mask size.

The proximity effects on the gate pattern can be compensated for byvarious techniques other than the extension of the extensions. Forexample, where gates are laid out with relatively wide space interposedtherebetween, a hammerhead pattern may be added to each extension of agate pattern that is not located over the active layer of a transistor.This hammerhead pattern does not extend the extension in the gate widthdirection, but expands only the edges of the extension in the gatelongitudinal direction, thereby preventing the actual size of the gatepattern from shrinking too much at its edges in the gate widthdirection. In this manner, the proximity effect corrections can be madenot only by compensating for the deviation of the actual size from themask size but also by minimizing the deviation. Accordingly, if the sizedeviation was simply expected and a design rule was defined based on theresult without estimating how much the deviation can be reduced byforming an OPC pattern, then the mask size determined would beunnecessarily large.

(2) In general, circuit patterns are made in accordance with a basicpattern placement rule and process conditions are defined to minimize avariation in sizes of patterns actually formed and a deviation of eachactual size from its mask size. However, another pattern placement rule,which is different from that applied to the definition of the processconditions, is applied to OPC patterns. Thus, the process conditionsdefined are not always best suited to the placement rule for the OPCpatterns.

Suppose circuit patterns have been defined with a pattern-to-patternspace minimized but the actual size of the space is larger than theoriginally designed one. In such a case, to make the size of the spacebetween OPC patterns smaller than that once defined for the circuitpatterns, the minimum space between the OPC patterns should be reducedfrom the minimum pattern-to-pattern space that was defined when theprocess conditions were determined. Accordingly, if the processconditions did not change at all, then the size of a pattern actuallyformed using the OPC pattern should almost match the size originallydesigned for the circuit pattern. Actually, though, the processconditions are variable during the fabrication process, thus making theactual size non-uniform due to the variation. This is because if anactual size is reduced, then optimum process conditions usually changeto minimize the size non-uniformity due to the variation of processconditions. In an extreme case, even a basic exposure method such assuper-resolution exposure or phase shift masking should be changed tosuppress the size nonuniformity.

(3) Although final process conditions for an LSI are determined justbefore the actual fabrication thereof, details of OPC patterns cannot bedefined until specific process conditions are determined.

In developing an LSI, design of circuit patterns to be registered at acell library is started as early as more than 6 months before thefabrication of the LSI. However, since the process conditions aredetermined just before the fabrication, details of OPC patterns cannotbe defined at an early stage. Accordingly, it is difficult to designcircuit patterns for a cell library in view of final OPC patterns tosolve the problem (1).

(4) An OPC pattern is made based on only a deviation of an actual sizeobtained under predetermined process conditions from a size originallydesigned for a circuit pattern. A design rule, which is ordinarilydefined such that a feature size of the previous generation should bereduced by 70%, is applied to the circuit pattern. Although the 70%shrinkage may be effectively applicable to some LSI's, other LSI's mayrequire a different shrinkage rate.

For example, in making an LSI with the same function and the same chiparea based on a new design rule, that LSI might be obtained at ashrinkage rate of 50%. Furthermore, in an actual circuit pattern, theactual size does not have to exactly match the originally designed onein every part of the circuit. That is to say, some parts of a circuitmay require an exact match between the actual and designed sizes toensure proper operation for the circuit, but other parts thereof mayallow some size deviation. Accordingly, if a circuit is designed suchthat all the actual sizes thereof are reduced by 70% compared to theolder generation, then unnecessarily stringent conditions will beimposed on the LSI fabrication, thus making it much harder to obtain adesired LSI.

SUMMARY OF THE INVENTION

An object of the present invention is performing optical proximitycorrections effectively in such a manner as to downsize an LSI just asintended and to form circuit patterns that will make the LSI fullyoperative.

To achieve this object, according to an inventive method for planning alayout for an LSI pattern or generating mask data for an LSI, opticalproximity corrected patterns are formed as mask data for circuitpatterns being designed. Also, a design rule is defined to make thecorrected patterns effectively applicable while the circuit patterns arebeing designed.

Specifically, an inventive method for planning a layout for an LSIpattern includes the steps of: designing circuit patterns included inthe LSI pattern; making an initial placement for the circuit patternsdesigned; performing optical proximity corrections on at least two ofthe circuit patterns that have been initially placed to be adjacent toor cross each other, thereby forming optical proximity correctedpatterns out of the adjacent or crossing circuit patterns; andevaluating effectiveness of the proximity corrections. If theeffectiveness of the corrections is negated, the method further includesthe steps of: changing a design rule defining the circuit patterns tomake the corrections effective; and making a re-placement for theinitially placed circuit patterns in accordance with the design rulechanged.

According to the inventive method for planning an LSI pattern layout,optical proximity corrected patterns are generated first, and then adesign rule defining circuit patterns is changed to make the correctionseffective. Thus, the present invention eliminates a situation, whereoptical proximity corrections are no longer applicable to a mask patternformed by transferring a designed pattern, as is often the case with theprior art.

In one embodiment of the present invention, the step of forming thecorrected patterns preferably includes the step of settingspecifications of the corrected patterns to be made. The step ofevaluating preferably includes the step of changing the specificationsto make the corrections effective if the effectiveness of thecorrections is negated.

In another embodiment of the present invention, the step of making there-placement preferably includes the step of forming multiplere-placement patterns and selecting one of the re-placement patternsthat minimizes a total circuit area.

In still another embodiment, the method may further include the step ofdefining the design rule such that the circuit patterns can be laid outto make the corrections effective. The step of making the initialplacement or the step of making the re-placement may include the step ofplacing the circuit patterns in accordance with the design rule.

In this case, the step of defining the design rule preferably includesthe step of defining multiple design rules and selecting one of thedesign rules that minimizes a total circuit area.

In an alternate embodiment, the method may further include the steps of:setting specifications of the corrected patterns to be made; defining arule of placing the corrected patterns such that the proximitycorrections can be performed effectively using the corrected patterns;and forming the corrected patterns according to the specifications ofthe corrected patterns to be made and on the rule of placing thecorrected patterns, thereby defining the design rule.

In this case, the method may further include the steps of: evaluatingeffectiveness of the proximity corrections for the circuit patterns thathave been placed in accordance with the design rule; and modifying thespecifications of the corrected patterns to be made or the rule ofplacing the corrected patterns to make the corrections effective if theeffectiveness of the corrections is negated.

In still another embodiment, the step of evaluating may include the stepof determining whether or not an expected actual size falls within apredetermined range by performing a process simulation including atleast one of lithography and etching process steps.

In this case, the lithography process step in the process simulationpreferably includes determining whether or not the expected actual sizefalls within the predetermined range even if an exposure dose or focushas changed so much as to exceed a process margin range.

Alternatively, it may be determined in the process simulation whether ornot an expected size of a transistor in a gate longitudinal directionfalls within the predetermined range.

As another alternative, it may also be determined in the processsimulation whether or not an expected length of a portion of atransistor gate extending from an active layer in a gate width directionfalls within the predetermined range.

A first inventive LSI pattern forming method includes the steps of: a)designing circuit patterns included in the LSI pattern; b) making aninitial placement for the circuit patterns designed; c) performingoptical proximity corrections on at least two of the circuit patternsthat have been initially placed to be adjacent to or cross each other,thereby forming optical proximity corrected patterns out of the adjacentor crossing circuit patterns; and d) evaluating effectiveness of theproximity corrections under predetermined process conditions. If theeffectiveness of the corrections is negated, then the method furtherincludes the steps of: e) changing a design rule defining the circuitpatterns to make the corrections effective; f) making a re-placement forthe initially placed circuit patterns in accordance with the design rulechanged; g) producing a mask using the corrected patterns; and h)defining the circuit patterns on a semiconductor substrate under thepredetermined process conditions by using the mask produced.

According to the first inventive LSI pattern forming method, circuitpatterns (i.e., actually transferred patterns) are formed on a resistfilm, for example, using a mask that has been produced by the inventiveLSI pattern layout planning method. Thus, these circuit patterns ensureproper operation for the resultant circuit.

In one embodiment of the present invention, the first LSI patternforming method may further include the step of estimating process yieldexpectedly attainable when the produced mask is used under thepredetermined process conditions after the step g) has been performed.And if the estimate is short of a target value, the method may furtherinclude the step of modifying the predetermined process conditions tomake the estimate reach the target value and repeatedly performing thesteps a) through g).

A first inventive LSI mask data generating method includes the step ofa) classifying multiple circuit patterns included in the LSI into firstand second groups of corrected patterns. Each said corrected pattern ofthe first group will not be changed in shape even when processconditions are modified, while each said corrected pattern of the secondgroup will be changed in shape if the process conditions are modified.The method further includes the steps of: b) generating cell-leveloptical proximity corrected pattern data from the first group ofcorrected patterns when the circuit patterns are designed; and c)generating chip-level optical proximity corrected pattern data from thesecond group of corrected patterns when chip data is generated from thecircuit patterns.

According to the first inventive LSI mask data generating method, evenif optical proximity corrections are made in advance on the first groupof corrected patterns, those patterns of the first group can beregistered at a library. Also, since the first group of correctedpatterns greatly affects cell areas, the corrected patterns aredetermined at a cell level, i.e., while the cells are being designed.Accordingly, the areas of the cells to be produced finally based on thecorrected patterns can be estimated accurately. Furthermore, since theoptical proximity corrections can be made on a cell-by-cell basis, thespecifications of optical proximity corrected patterns to be made can bedetermined for each cell or block.

In one embodiment of the present invention, the step b) may include thestep of evaluating effectiveness of optical proximity correctionsrepresented by the cell-level corrected pattern data generated. If theeffectiveness of the corrections is negated, the method may furtherinclude the step of modifying the cell-level corrected pattern data orcircuit patterns corresponding to the cell-level corrected pattern datato make the corrections effective and then re-evaluating effectivenessof the corrections. And if the effectiveness of the corrections isaffirmed, the method may include the step of registering the cell-levelcorrected pattern data at a cell library.

A second inventive LSI mask data generating method includes the step ofa) classifying multiple circuit patterns included in the LSI into firstand second groups of corrected patterns. Each said corrected pattern ofthe first group will not be changed in shape even when processconditions are modified, while each said corrected pattern of the secondgroup will be changed in shape if the process conditions are modified.The method further includes the steps of: b) setting specifications ofcell-level optical proximity corrected patterns to be made for the firstgroup of corrected patterns; c) designing the circuit patterns; and d)evaluating effectiveness of optical proximity corrections if thecell-level corrected patterns, which have been made for the first groupof corrected patterns to the specifications of the cell-level correctedpatterns, are used. If the effectiveness of the corrections is negated,the method further includes the step of e) modifying ineffective circuitpatterns to make the corrections effective and re-evaluatingeffectiveness of the corrections. And if the effectiveness of thecorrections is affirmed, the method further includes the steps of: f)registering the circuit patterns, belonging to the first and secondgroups of corrected patterns, at a cell library; g) generatingchip-level pattern data from the circuit patterns that have beenregistered at the cell library; h) setting specifications of chip-leveloptical proximity corrected patterns to be made for the second group ofcorrected patterns; i) generating cell-level optical proximity correctedpattern data from the circuit patterns belonging to the first group ofcorrected patterns according to the specifications of the cell-levelcorrected patterns; and j) generating chip-level optical proximitycorrected pattern data from the circuit patterns belonging to the secondgroup of corrected patterns according to the specifications of thechip-level corrected patterns.

According to the second inventive LSI mask data generating method,specifications of cell-level corrected patterns are set for the circuitpatterns belonging to the first group of corrected patterns, and thosecircuit patterns are registered at a cell library. Thereafter, when maskdata is generated, cell- and chip-level corrected patterns with a hugedata volume are made using the cell library. Accordingly, that hugevolume of data can be managed more easily.

A third inventive LSI mask data generating method includes the step ofa) classifying multiple circuit patterns included in the LSI into firstand second groups of corrected patterns. Each said corrected pattern ofthe first group will not be changed in shape even when processconditions are modified, while each said corrected pattern of the secondgroup will be changed in shape if the process conditions are modified.The method further includes the steps of: b) setting specifications ofcell-level optical proximity corrected patterns to be made for the firstgroup of corrected patterns; c) designing the circuit patterns; and d)evaluating effectiveness of optical proximity corrections if thecell-level corrected patterns, which have been made for the first groupof corrected patterns to the specifications of the cell-level correctedpatterns, are used. If the effectiveness of the corrections is negated,the method includes the step of e) modifying ineffective circuitpatterns or the specifications of the cell-level corrected patternscorresponding to the circuit patterns to make the corrections effectiveand re-evaluating effectiveness of the corrections. And if theeffectiveness of the corrections is affirmed, the method furtherincludes the steps of: f) registering not only the circuit patternsbelonging to the first group of corrected patterns and thespecifications of the cell-level corrected patterns corresponding to thecircuit patterns, but also the circuit patterns belonging to the secondgroup of corrected patterns at a cell library; g) generating chip-levelpattern data from the circuit patterns that have been registered at thecell library; h) generating cell-level optical proximity correctedpattern data from the circuit patterns belonging to the first group ofcorrected patterns according to the specifications of the cell-levelcorrected patterns; and i) generating chip-level optical proximitycorrected pattern data from the circuit patterns belonging to the secondgroup of corrected patterns according to the specifications of thechip-level corrected patterns.

In one embodiment of the first through third inventive LSI mask datagenerating methods, if the corrections are effectively applicable tomultiple layouts for circuit patterns, the step of evaluating preferablyincludes the step of selecting one of the layouts that results in acircuit area equal to or smaller than a predetermined value.

In this particular embodiment, the cell-level corrected pattern datapreferably includes serif patterns, hammerhead patterns or in-sectionpatterns.

A fourth inventive LSI mask data generating method includes the step ofa) classifying multiple circuit patterns included in the LSI into firstand second groups of corrected patterns. Each said corrected pattern ofthe first group is defined by circuit patterns placed to cover multiplelayers, while each said corrected pattern of the second group is definedby circuit patterns placed within a single layer. The method furtherincludes the steps of: b) generating interlayer optical proximitycorrected pattern data from the first group of corrected patterns indesigning the circuit patterns; and c) generating intralayer opticalproximity corrected pattern data from the second group of correctedpatterns in generating chip data from the circuit patterns.

According to the fourth inventive LSI mask data generating method, evenif optical proximity corrections are made in advance on the first groupof corrected patterns, those patterns of the first group can beregistered at a library. Also, since the first group of correctedpatterns greatly affects cell areas, the corrected patterns aredetermined at a cell level, i.e., while the cells are being designed.Accordingly, the areas of the cells to be produced finally based on thecorrected patterns can be estimated accurately. Furthermore, since theoptical proximity corrections can be made on a cell-by-cell basis, thespecifications of optical proximity corrected patterns to be made can bedetermined for each cell or block.

In one embodiment of the present invention, the step b) may include thestep of evaluating effectiveness of optical proximity correctionsrepresented by the interlayer corrected pattern data generated. If theeffectiveness of the corrections is negated, the method further includesthe step of modifying the interlayer corrected pattern data or thecircuit patterns corresponding to the corrected pattern data to make thecorrections effective and re-evaluating effectiveness of thecorrections. And if the effectiveness of the corrections is affirmed,the method further includes the step of registering the interlayercorrected pattern data at a cell library.

A fifth inventive LSI mask data generating method includes the step ofa) classifying multiple circuit patterns included in the LSI into firstand second groups of corrected patterns. Each said corrected pattern ofthe first group is defined by circuit patterns placed to cover multiplelayers, while each said corrected pattern of the second group is definedby circuit patterns placed within a single layer. The method furtherincludes the steps of: b) setting specifications of interlayer opticalproximity corrected patterns to be made for the first group of correctedpatterns; c) designing the circuit patterns; and d) evaluatingeffectiveness of optical proximity corrections if the interlayercorrected patterns, which have been made for the first group ofcorrected patterns to the specifications of the interlayer correctedpatterns, are used. If the effectiveness of the corrections is negated,the method further includes the step of e) modifying ineffective circuitpatterns to make the corrections effective and re-evaluatingeffectiveness of the corrections. And if the effectiveness of thecorrections is affirmed, the method further includes the steps of: f)registering the circuit patterns, belonging to the first and secondgroups of corrected patterns, at a cell library; g) generatingchip-level pattern data from the circuit patterns that have beenregistered at the cell library; h) setting specifications of intralayeroptical proximity corrected patterns to be made for the second group ofcorrected patterns; i) generating interlayer optical proximity correctedpattern data from the circuit patterns belonging to the first group ofcorrected patterns according to the specifications of the interlayercorrected patterns; and j) generating intralayer optical proximitycorrected pattern data from the circuit patterns belonging to the secondgroup of corrected patterns according to the specifications of theintralayer corrected patterns.

According to the fifth inventive LSI mask data generating method,specifications of interlayer corrected patterns are set for the circuitpatterns belonging to the first group of corrected patterns, and thosecircuit patterns are registered at a cell library. Thereafter, when maskdata is generated, interlayer and intralayer corrected patterns with ahuge data volume are made using the cell library. Accordingly, that hugevolume of data can be managed more easily.

A sixth inventive LSI mask data generating method includes the step ofa) classifying multiple circuit patterns included in the LSI into firstand second groups of corrected patterns. Each said corrected pattern ofthe first group is defined by circuit patterns placed to cover multiplelayers, while each said corrected pattern of the second group is definedby circuit patterns placed within a single layer. The method furtherincludes the steps of: b) setting specifications of interlayer opticalproximity corrected patterns to be made for the first group of correctedpatterns; c) designing the circuit patterns; and d) evaluatingeffectiveness of optical proximity corrections if the interlayercorrected patterns, which have been made for the first group ofcorrected patterns to the specifications of the interlayer correctedpatterns, are used. If the effectiveness of the corrections is negated,the method includes the step of e) modifying ineffective circuitpatterns or the specifications of the interlayer corrected patternscorresponding to the circuit patterns to make the corrections effectiveand re-evaluating effectiveness of the corrections. And if theeffectiveness of the corrections is affirmed, the method furtherincludes the steps of: f) registering not only the circuit patternsbelonging to the first group of corrected patterns and thespecifications of the interlayer corrected patterns corresponding to thecircuit patterns, but also the circuit patterns belonging to the secondgroup of corrected patterns at a cell library; g) generating chip-levelpattern data from the circuit patterns that have been registered at thecell library; h) generating interlayer optical proximity correctedpattern data from the circuit patterns belonging to the first group ofcorrected patterns according to the specifications of the interlayercorrected patterns; and i) generating intralayer optical proximitycorrected pattern data from the circuit patterns belonging to the secondgroup of corrected patterns according to the specifications of theintralayer corrected patterns.

In one embodiment of the fourth through sixth inventive LSI mask datagenerating methods, if the corrections are effectively applicable tomultiple layouts for circuit patterns, the step of evaluating mayinclude the step of selecting one of the layouts that results in acircuit area equal to or smaller than a predetermined value.

In this particular embodiment, the specifications of the interlayercorrected patterns to be made are preferably determined by a placementrule defining a layer including a gate of a transistor and another layerincluding an active region.

Alternatively, the specifications of the interlayer corrected patternsto be made may also be determined by a placement rule defining a firstinterconnection layer and another layer including a contact forelectrically connecting the first interconnection layer to a secondinterconnection layer.

As another alternative, the step of evaluating may include the step ofdetermining whether or not an expected actual size falls within apredetermined range by performing a process simulation including atleast one of lithography and etching process steps.

In this case, the lithography process step in the process simulation mayinclude determining whether or not the expected actual size falls withinthe predetermined range even if an exposure dose or focus has changed somuch as to exceed a process margin range.

In this case, it may be determined in the process simulation whether ornot an expected size of a transistor in a gate longitudinal directionfalls within the predetermined range.

Alternatively, it may also be determined in the process simulationwhether or not an expected length of a portion of a transistor gateextending from an active layer in a gate width direction falls withinthe predetermined range.

Second through seventh inventive methods for forming an LSI pattern eachinclude the steps of: producing a mask in accordance with any of thefirst through sixth LSI mask data generating methods of the presentinvention; and defining the circuit patterns on a semiconductorsubstrate using the mask produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating an LSI mask data generating methodaccording to a first embodiment of the pre-sent invention.

FIG. 2 is a flowchart illustrating an LSI pattern layout planning methodaccording to the first embodiment.

FIG. 3 is a plan view illustrating exemplary circuit patterns used inthe LSI pattern layout planning method according to the firstembodiment.

FIG. 4 is a plan view illustrating exemplary OPC pattern that have beenmade from the circuit patterns shown in FIG. 3 in the LSI pattern layoutplanning method according to the first embodiment.

FIG. 5 is a plan view illustrating another exemplary OPC patterns thathave also been made from the circuit patterns shown in FIG. 3 in the LSIpattern layout planning method according to the first embodiment.

FIG. 6 is a plan view illustrating exemplary actually transferredpatterns that have been made from the OPC patterns shown in FIG. 5 inthe LSI pattern layout planning method according to the firstembodiment.

FIG. 7 is a plan view illustrating modified versions of the circuitpatterns shown in FIG. 3 in the LSI pattern layout planning methodaccording to the first embodiment.

FIG. 8 is a plan view illustrating exemplary OPC patterns that have beenmade from the circuit patterns shown in FIG. 7 in the LSI pattern layoutplanning method according to the first embodiment.

FIG. 9 is a plan view illustrating exemplary actually transferredpatterns that have been made from the OPC patterns shown in FIG. 8 inthe LSI pattern layout planning method according to the firstembodiment.

FIGS. 10 and 11 illustrate a flowchart of an LSI mask data generatingmethod according to a second embodiment of the present invention.

FIGS. 12( a) and 12(b) illustrate patterns used to exemplify categoriesadopted in the LSI mask data generating method according to a secondembodiment:

FIG. 12( a) is a plan view illustrating chip-level circuit patternsbelonging to a second category; and

FIG. 12( b) is a plan view illustrating cell-level circuit patternsbelonging to a first category.

FIGS. 13 and 14 illustrate a flowchart of an LSI mask data generatingmethod according to a modified example of the second embodiment.

FIGS. 15 and 16 illustrate a flowchart of an LSI mask data generatingmethod according to a third embodiment of the present invention.

FIGS. 17( a) and 17(b) illustrate patterns used to exemplify categoriesadopted in the LSI mask data generating method according to the thirdembodiment:

FIG. 17( a) is a plan view illustrating interlayer circuit patternsbelonging to a first category; and

FIG. 17( b) is a plan view illustrating exemplary OPC patterns that havebeen made from the circuit patterns shown in FIG. 17( a).

FIGS. 18 and 19 illustrate a flowchart of an LSI mask data generatingmethod according to a modified example of the third embodiment.

FIGS. 20( a) and 20(b) are plan views illustrating originally designedand actually transferred patterns for a transistor according to aconventional LSI mask data generating method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

Hereinafter, a first embodiment of the present invention will bedescribed with reference to the accompanying drawings. According to thefirst embodiment, a design rule is determined so as to satisfy theconditions that should be met to form OPC patterns of the type makingoptical proximity corrections effective when an LSI is fabricated. Andcircuit patterns are designed and mask pattern data is generated inaccordance with the design rule determined.

Also, the OPC patterns are made from the circuit patterns in accordancewith the design rule that makes the OPC effective, and basic processconditions are optimized based on the OPC patterns obtained. In thisspecification, the OPC can be regarded as “effective” when a pattern,which has been actually transferred onto a region with substantially thesame area as an originally designed circuit pattern using this OPCpattern, ensures proper operation for a resulting circuit.

FIG. 1 illustrates a processing flow of an LSI mask data generatingmethod according to the first embodiment. As shown in FIG. 1, first, inStep SA1, design rules that should be met to design patterns forcircuits to be included in an LSI, basic process conditions,specifications of OPC patterns to be made from the circuit patterns andOPC pattern placement rules are defined. In this case, the “basicprocess conditions” include the wavelength, coherency, focal point anddose of exposing radiation and the numerical aperture of a lens as for alithographic process. The basic process conditions further includeselection of an exposure method, e.g., whether or not annularillumination technique should be adopted and whether or not a phaseshifted mask should be used. The “design rules” are rules that should bemet in the design of circuit patterns to obtain an actually operativecircuit. The “OPC placement rules” are rules that should be met by OPCpatterns such that the pattern actually transferred onto a wafer throughexposure is processable. In other words, these rules are “design rules”imposed on the OPC patterns so to speak. And these rules include variousdetails defining a basic pattern placement such as the minimum linewidth or space width of the OPC patterns. In this manner, design rulesthat will make originally designed patterns actually transferable arealso applicable to OPC patterns to be mask patterns, while anotherdesign rules, which have been defined taking the OPC patterns to beformed into account, are applicable to circuit patterns. Thus, optimumconditions can be selected as the basic process conditions after the OPCpatterns have been defined.

Next, in Step SA2, circuit patterns are formed on a cell-by-cell basis.In this case, the “cell” is a basic circuit component of an LSI.

Subsequently, in Step SA3, it is determined whether or not each of thecircuit patterns that have been formed in Step SA2 meets the designrules. If the answer is NO, the method proceeds to Step SA4 of modifyingpart of the circuit pattern not meeting the design rules, and then StepsSA2 and SA3 are repeatedly performed. Alternatively, if the answer isYES, the method proceeds to Step SA5.

Then, in Step SA5, the circuit patterns that have been formed on acell-by-cell basis are registered at a cell library, thereby storingbasic cells needed to make an LSI chip pattern.

Thereafter, in Step SA6, circuit pattern data required for the LSI isextracted from the cell library and LSI chip data is generated using thecircuit pattern data extracted.

Next, in Step SA7, final process conditions for the realization of theLSI chip data are determined. In this case, if the OPC placement rulesshould be changed in view of the final process conditions, then the OPCplacement rules are changed. The reason is as follows. Generallyspeaking, in developing an LSI, it takes more than one year to establisha needed cell library after design rules were defined. In contrast, ittakes at most several months to generate LSI chip data after the celllibrary was established. Accordingly, even if optimum process conditionshad been determined to meet the design rules, the process conditionsmight be no longer optimum because of the introduction of new resistmaterials or apparatuses when LSI chip data should be generated using acell library established. Thus, to further improve the productivity, thefinal process conditions should preferably be redefined when LSI chipdata is generated.

Subsequently, in Step SA8, required OPC patterns are made based on theLSI chip data and according to the specifications of the OPC patterns tobe made. Specifically, a deviation of an actual size from a mask sizedue to optical proximity effects is estimated under the final processconditions. And data is obtained about a mask layout that has beenmodified in such a manner as to eliminate the deviation of the actualsize from the originally designed one.

Then, in Step SA9, it is determined whether or not each of the OPCpatterns that have been made in Step SA8 meets the OPC pattern placementrules. If the answer is NO, the method proceeds to Step SA10 ofcorrecting part of the OPC pattern that does not meet the OPC patternplacement rules, and then Steps SA8 and SA9 are repeatedly performed.Alternatively, if the answer is YES, the method proceeds to Step SA11 ofgenerating mask pattern data using the OPC patterns.

Based on the mask pattern data obtained in this manner, a mask orreticle is produced and a pattern that will make a resulting circuitfully operative can be transferred onto a resist film on a semiconductorsubstrate, for example.

As described above, according to the prior art LSI development process,design rules are determined in an earlier process step and OPC patternsare formed in a later process step. Thus, if there is a circuit patternor its placement, from which no OPC patterns can be made, it isvirtually impossible to change the design rules. In contrast, accordingto the present invention, even after the design rules have been oncedefined, the design rules are modifiable in such a manner as to make theOPC patterns effectively applicable. Accordingly, circuit patterns ormask data that has been generated in accordance with the modified designrules can make the OPC effective with much more certainty.

Hereinafter, the details of the process step SA1 shown in FIG. 1 will bedescribed with reference to FIG. 2.

FIG. 2 illustrates an LSI pattern layout planning method according tothe first embodiment. Specifically, FIG. 2 illustrates an exemplaryprocedure of determining the basic process conditions and design rules,which are applied to the cell library. As shown in FIG. 2, first, inStep SB1, the design rules, basic process conditions and OPC patternplacement rules to be determined by the basic process conditions areinitialized. These initial values are set to obtain several typicalsamples for the cell library to be established in Step SA6 shown in FIG.1.

Next, in Step SB2, circuit patterns are formed in accordance with thedesign rules defined.

Subsequently, in Step SB3, it is determined whether or not each of thecircuit patterns formed in Step SB2 meets the design rules. If theanswer is NO, the method proceeds to Step SB4 of modifying part of thecircuit pattern not meeting the design rules, and then Steps SB2 and SB3are repeatedly performed.

Then, in Step SB5, specifications of OPC patterns to be made are setwith respect to the circuit patterns. The OPC pattern specifications maybe set by any known technique, i.e., no matter whether the technique isrules-based or models-based (i.e., simulation-based). That is to say, ifthe same circuit pattern should be made, then any technique isapplicable so long as the same OPC pattern can be obtained by thetechnique. According to the “rules-based” technique, rules of making anOPC pattern are defined for each pattern category of the circuitpattern, and the OPC pattern is formed following the rules defined. Onthe other hand, according to the “models-based” technique, a maskpattern size is calculated by a model equation, which is used tosimulate an actual pattern size, such that the actual pattern sizematches an originally designed one in a circuit pattern.

Thereafter, in Step SB6, OPC patterns are formed for respective circuitpatterns to the OPC pattern specifications that have been set in StepSB5.

Hereinafter, specific examples of the circuit and OPC patterns will bedescribed with reference to the accompanying drawings.

FIG. 3 illustrates exemplary circuit patterns. As shown in FIG. 3, thecircuit patterns representing a transistor circuit includes a rectangleactive layer pattern 11, which has a notched longer side. Over theactive layer pattern 11, first, second and third gate patterns 12, 13and 14 are placed. The first gate pattern 12 crosses the two longersides of the active layer pattern 11 and overlaps with non-notched partof the active layer pattern 11. The second and third gate patterns 13and 14 are both parallel to the first gate pattern 12 and both overlapwith the notched part of the active layer pattern 11. In addition, aline pattern 15 is also placed to be spaced apart from, and parallel to,the notched longer side of the active layer pattern 11.

The third gate pattern 14 includes a transistor portion 14 a functioningas gate electrode of the transistor and a gate line portion 14 b with afolded part, which extends over a peripheral region (i.e., isolationregion) of the active layer pattern 11.

FIG. 4 illustrates exemplary OPC patterns that have been made from thecircuit patterns shown in FIG. 3. In the illustrated example, the OPCpattern specifications are set in the following manner. First,hammerhead patterns 12 h, 13 h and 14 h are provided for the respectiveedges of the first, second and third gate patterns 12, 13 and 14 thatare closer to the first line pattern 15. Also, the widths (i.e., gatelengths) of respective parts of the gate patterns 12, 13 and 14 that arelocated over the active layer pattern 11 are changed in accordance withthe distance between an adjacent pair of gate patterns.

Next, in Step SB7, it is determined whether or not each of the OPCpatterns that have been made in Step SB6 shown in FIG. 2 meets the OPCpattern placement rules set in Step SB1. For instance, in the exampleillustrated in FIG. 4, the width of the space between the patterns 13 hand 14 h in a region 17 to be checked is smaller than the minimumpermissible space width as defined by the OPC pattern placement rules.If the OPC pattern placement rules are not met in this manner, then themethod proceeds to Step SB8 shown in FIG. 2. In Step SB8, the OPCpattern specifications are modified such that the OPC pattern placementrules are also met in the region 17. And then Steps SB5, SB6 and SB7 arerepeatedly performed. To eliminate the violation of the rule in theregion 17 shown in FIG. 4, the specifications should be modified bychanging the shapes of the respective OPC patterns 12 through 14depending on the distance between adjacent patterns, e.g., thehammerhead patterns 12 h, 13 h and 14 h. FIG. 5 illustrates OPC patternsthat have been re-shaped according to the modified OPC patternspecifications. As encircled in the region 17 shown in FIG. 5, thehammerhead pattern 13 h is erased from the edge of the second gatepattern 13 closer to the line pattern 15 and that edge of the secondgate pattern 13 is extended and aligned with those of the hammerheadpatterns 12 h and 14 h of the first and third gate patterns 12 and 14.

After it has been confirmed that all the OPC pattern placement rules aresatisfied, it is determined whether or not the dimensions of an actualpattern that will be made from the OPC pattern, i.e., criticaldimensions (CD), will match those of the originally designed circuitpattern in Step SB9 shown in FIG. 2. In other words, Step SB9 is aprocess step of seeing if an effective OPC pattern has been formed. Inthis case, it is usually difficult to determine, using a real circuit,whether or not the dimensions of the actually transferred pattern matchthose of the originally designed circuit pattern. Thus, some simulationtechnique that can reproduce the real circuit precisely should be used.It should be noted, however, that it is not necessary to check the CDfor every part of a single circuit. In actuality, the CD's has to bechecked in only a portion of a circuit, where the actual size shouldexactly match the designed size, e.g., the gate length of a gatepattern. If it has been determined that the CD's are not equal to eachother, then the method proceeds to Step SB8, in which the OPC patternspecifications are further modified to eliminate the portions inquestion from the OPC pattern. Then, Steps SB5, SB6, SB7 and SB9 arerepeatedly performed.

Next, in Step SB10, it is determined whether or not the OPC pattern withits CD's checked is effective. In this case, it is not whether the sizeof the actual pattern exactly matches that of the originally designedpattern that should be determined. Instead it has to be determinedwhether the actual pattern size meets the conditions that should besatisfied to make the circuit operate properly. In Step SB10, somesimulation technique that can reproduce a real circuit precisely is alsoused as in Step SB9. For example, the actual size of a gate extension ofa circuit pattern is checked. Specifically, it is not whether the actualsize of an edge of a gate pattern matches that of the originallydesigned circuit pattern that should be determined here. Instead, it maybe determined whether or not an active layer pattern is exposed in aregion where the active layer pattern overlaps with the gate pattern inthe actually transferred pattern as a result of the disappearance of theextension of the gate pattern. Also, even if the size of the extensionof the actually formed gate pattern is longer than a predetermined size,there is no problem unless the operation of the circuit is interferedwith a shortcircuit caused between the elongated extension and anotherpattern. However, the effectiveness of the OPC cannot be checked toocarefully because the circuit is inoperative anymore should anyconvenience be caused. Thus, in checking the effectiveness, not only thepredetermined process conditions, but also process conditions withmargins should be taken into account because the process conditions arevariable during real-world fabrication process.

FIG. 6 illustrates exemplary simulated patterns that would be actuallyformed and are analyzed in Step SB10 of checking the effectiveness ofthe OPC. As shown in FIG. 6, all of the corners and notch of the activelayer pattern 11A have been rounded and the extension of the second gatepattern 13 closer to the line pattern 15 has almost disappeared. As canbe seen from this simulation result, the gate width of the second gatepattern 13A has shortened in a first region 17A to be checked and thesource/drain regions of the active layer pattern 11A of the transistorare virtually short-circuited, thus preventing the circuit fromoperating normally. In addition, in a second region 18A to be checked,the folded portion of the third gate pattern 14A has also been roundedand the gate length has locally increased near a side of the activelayer pattern 11A, thus also interfering with the proper operation. Inthe example shown in FIG. 6, however, no margin has been added to theprocess conditions. In practice, though, an actually formed pattern issimulated after predetermined margin has been added to the processconditions.

If the effectiveness of the OPC is negated as shown in FIG. 6 (i.e., ifit has been determined that the circuit would not operate properly),then the method proceeds to Step SB11 shown in FIG. 2. In Step SB11, itis determined whether or not there is any pattern placement that makesthe OPC ineffective in the circuit patterns.

If the answer to the query in Step SB11 is NO, then Steps SB8, SB5, SB6,SB7, SB9 and SB10 are repeatedly performed to modify the OPC patternspecifications such that the OPC becomes effective. Alternatively, ifthe answer to the query in Step SB11 is YES, then the method proceeds toStep SB12, in which the design rules are modified to eliminate thepattern placement that makes the OPC ineffective. Then, respective stepsfrom SB4 on are repeatedly performed.

FIG. 7 illustrates a result of Step SB4 of modifying the patternplacement that makes the OPC ineffective. In the illustrated example,the design rules are modified such that a predetermined space isprovided between the gate and active layer patterns. Accordingly, aspace between a longer side of the active layer pattern 11B and a partof the gate line portion 14 b of the third gate pattern 14B that isparallel to the longer side of the active layer pattern 11B is increasedcompared to its initial value. In the same way, the space between a sideof the second gate pattern 13 and an edge of the notch of the activelayer pattern 11B between the first and second gate patterns 12 and 13is also larger than its initial value. In FIG. 7, the original outlinesof the third gate pattern 14 and the active layer pattern 11 before themodification are represented by broken lines.

FIG. 8 illustrates exemplary OPC patterns that have been made from thecircuit patterns shown in FIG. 7. FIG. 9 illustrates simulated patternsthat would be made actually from the OPC patterns shown in FIG. 8. Asshown in FIG. 9, the extension of the second gate pattern 13A at itsedge closer to the line pattern 15A is long enough to ensure apredetermined gate length. Also, the gate length of the transistorportion 14 a of the third gate pattern 14C is substantially constant. Ascan be seen, by changing the design rules in such a manner as to makethe OPC effective, circuit patterns ensuring effective OPC can beobtained without repeating several process steps all over again.

Then, in Step SB13 shown in FIG. 2, a total circuit area (i.e., cellarea) of the circuit patterns ensuring the OPC effects is estimated. Inaddition, an expected yield of circuits that can be formed based on theactual patterns corresponding to the OPC versions of the circuitpatterns and that would operate normally is also estimated. The expectedyield may be estimated by a technique of expecting the percentage oftransistors operating normally in a cell as disclosed in JapaneseLaid-Open Publication No. 10-284608 or No. 11-121345. This is becausethe percentage of normally operating transistors can be regarded as theexpected yield of the circuit patterns. More specifically, an actualsize ensuring normal operation of transistors is represented as aresponse surface function using the process conditions or the size of amask pattern for a transistor as variables. Furthermore, a distributionof expected variation in process conditions during a fabrication processis substituted into the response surface function, thereby calculatingthe probability of normal operation of transistors when the actual sizeis adopted in the fabrication process. In general, the smaller the totalarea of circuit patterns, the lower the expected yield of circuitsoperating normally.

If it has been determined in Step SB13 that the total area of thecircuit patterns is larger than the originally designed one, then themethod proceeds to Step SB14, in which the design rules andcorresponding OPC pattern placement rules are modified to obtain smallercircuit patterns. Then, respective steps from Step SB1 on are performedall over again. In the same way, if it has been determined in Step SB13that the expected yield of normally operating circuits is lower than itstarget value, then the method also advances to Step SB14, in which thebasic process conditions are improved and the design rules andassociated OPC pattern placement rules are changed to obtain largercircuit patterns. Then, respective steps from Step SB1 on are performedall over again.

On the other hand, if both the total area of the circuit patterns andthe expected yield meet are within their target ranges, then the methodproceeds to Step SB15. In this process step SB15, the design rules,basic process conditions, OPC pattern placement rules, OPC patternspecifications and circuit pattern data are all determined finally.

As described above, according to this embodiment, multiple circuits (orcells) are prepared as typical samples to be registered at a celllibrary, thereby realizing a target circuit area of the cell library ofthe current generation. In addition, the basic process conditions anddesign rules can be determined for the resulting circuit so as to attainthe expected value of normal operation. Naturally, the larger the numberof samples, the better the design rules, OPC pattern placement rules andOPC pattern specifications can be.

In summary, the effects attainable by this embodiment are as follows:

(a) Design rules are determined to meet the conditions that should besatisfied to make the OPC effective. And circuit patterns are designedin accordance with the design rule determined. Thus, required OPCpatterns can always be obtained before the final process step ofgenerating mask pattern data.

(b) Design rules are finally determined such that the conditions thatshould be met to make the OPC effective are also reflected on designrules defined to design circuit patterns belonging to several typicalcategories. Thus, the design rules obtained are applicable very broadly.

(c) In determining the design rules, a predetermined cell area isadopted as one of the conditions that make the OPC effective. Thus, ifthe predetermined cell area is set to half as large as that of thecircuits included in an LSI of the previous generation, then the designrules cannot be larger than required.

(d) Design rules are decreased based on the shrinkage rate of a totalcircuit area, not based on that of the size of an actual pattern that ismade from each circuit pattern defined by the design rules. Thus, apattern designed is not unnecessarily hard to realize.

(e) The basic process conditions are redefined in view of OPC patternsto be made and to improve the productivity as in Step SB14. Thus, thebasic process conditions are not defined inappropriately compared tofinal process conditions.

Embodiment 2

Hereinafter, a second embodiment of the present invention will bedescribed with reference to the accompanying drawings. According to thesecond embodiment, design rules are determined so as to satisfy theconditions that should be met to form required OPC patterns as in thefirst embodiment. And circuit patterns are designed and mask patterndata is generated in accordance with the design rules determined. Inparticular, according to the second embodiment, each cell (basiccircuit) being designed can be individually checked for theeffectiveness of the OPC. Thus, a cell area can be reduced on acell-by-cell basis.

FIGS. 10 and 11 illustrate a processing flow of an LSI mask datagenerating method according to the second embodiment.

First, in Step SC1 shown in FIG. 10, design rules, basic processconditions and OPC pattern placement rules are determined. In this case,the design rules and the basic process conditions are determined as inStep SA1 shown in FIG. 1. On the other hand, the OPC pattern placementrules are defined by classifying OPC patterns into first and secondcategories. Each OPC pattern in the first category does not have to bemodified so sensitively to any variations in process conditions. Incontrast, each OPC pattern in the second category does have to bemodified sensitively to the variations in process conditions.

Hereinafter, specific examples of the first and second categories willbe described with reference to FIGS. 12( a) and 12(b). As shown in FIG.12( a), first and second patterns 21A and 22A are placed and spacedapart from each other. Each of the first and second patterns 21A and 22Aincludes a line portion 21 a, 22 a and an extension 21 b, 22 b thatextends from one side of the line portion 21 a, 22 a. The end of theextension 21 b of the first pattern 21A reaches farther than that of theextension 22 b of the second pattern 22A. Also, these line portions 21 aand 22 a are placed in parallel to each other. In this case, if an OPCpattern should be formed with the actual line width of the line portion21 a or 22 a changed, then the OPC pattern should be sensitive to anyvariations in process conditions and is classified in the secondcategory. Although not shown, the actual gate length of a gate patternhas to exactly match its originally designed size and is also classifiedin the second category.

On the other hand, a first OPC pattern 21B is formed based on the firstpattern 21A as shown in FIG. 12( b). The first OPC pattern 21B includes:a hammerhead pattern 21 c provided at the end of the extension 21 b; andan in-section pattern 21 d formed by thinning the connection between theline and extensions 21 a and 21 b. The hammerhead pattern 21 c and thein-section pattern 21 d do not have to be modified sensitively tovariations in process conditions, and are classified as the firstcategory. Similarly, serif patterns 22 c provided at both corner edgesof the extension 22 b of a second OPC pattern 22B, which has been formedbased on the second pattern 22A, are also classified as the firstcategory. In the illustrated example, the hammerhead and serif patterns21 c and 22 c are provided to prevent the edges of the original patternsfrom disappearing, while the in-section pattern 21 d is provided toprevent the connection between the line and extensions 21 a and 21 bfrom rounding.

Generally speaking, an OPC pattern of the type playing a key role indetermining the total area of circuit patterns (i.e., a cell area), oran OPC pattern attaining remarkable OPC effects for a miniaturized cellarea, belongs to the first category. In the following description, suchan OPC pattern in the first category will be called a “cell-level OPCpattern”, because optical proximity corrections can be performed usingsuch a pattern even at the stage of designing a cell library beforeprocess conditions are finally determined. In contrast, an OPC patternin the second category cannot be formed until process conditions arefinally determined. That is to say, optical proximity corrections can beperformed using such a pattern only after LSI chip data has beengenerated and final process conditions have been defined. Thus, apattern of the latter type will be called a “chip-level OPC pattern”.

Next, in Step SC2 shown in FIG. 10, circuit patterns are formed on acell-by-cell basis in accordance with the design rules defined in StepSC1 irrespective of the categories of the patterns.

Subsequently, in Step SC3, it is determined whether or not each of thecircuit patterns formed meets the design rules. If the answer is NO, themethod proceeds to Step SC4 of modifying part of the circuit pattern notmeeting the design rules, and then Steps SC2 and SC3 are repeatedlyperformed. Alternatively, if the answer is YES, the method proceeds toStep SC5.

Then, in Step SC5, cell-level OPC patterns are made from some of thecircuit patterns that belong to the first category. Each cell-level OPCpattern is preferably formed by a rules-based technique. Specifically,rules that should be followed in making a cell-level OPC pattern fromeach of the circuit patterns are defined, and then the cell-level OPCpattern is made in accordance with the rules defined. In this case, itis to make OPC fully effective that an OPC pattern is made. Accordingly,an OPC pattern does not have to be made such that an estimated size ofan actual pattern exactly matches the size of an originally designedcircuit pattern. Instead, the OPC pattern should be optimized to obtaina simulated actual pattern allowing the circuit pattern to operatenormally within its limited area. That is to say, any OPC pattern may beformed so long as the OPC pattern would contribute to increasing anexpected yield of normally operating circuits. More specifically, theestimated actual size of a portion that does not interfere with thenormal operation of the circuit may greatly deviate from that of acorresponding portion of the circuit pattern. Accordingly, a rules-basedtechnique, by which rules that should be followed in making an OPCpattern can be defined for each feature of a circuit pattern, issuitably applicable as a model of forming the OPC pattern. This isbecause estimated actual sizes determined for a circuit pattern areeffectuated as they are according to the models-based technique.

Then, in Step SC6, it is determined whether or not each of thecell-level OPC patterns that have been made in Step SC5 meets the OPCpattern placement rules. If the answer is NO, the method proceeds toStep SC7 of modifying part of the cell-level OPC pattern that does notmeet the OPC pattern placement rules, and then Steps SC5 and SC6 arerepeatedly performed.

Next, in Step SC8, it is determined whether or not OPC attainable byeach of the cell-level OPC patterns meeting the OPC pattern placementrules is effective. The effectiveness of the OPC may be evaluated bysome simulation technique that can reproduce a real circuit precisely asin Step SB10 according to the first embodiment. Specifically, it is notwhether the estimated actual size of a gate pattern edge matches that ofthe originally designed circuit pattern that should be determined.Rather, it should be determined whether or not an active layer patternis exposed in a region where the active layer pattern overlaps with thegate pattern in the actually transferred pattern as a result of thedisappearance of the extension of the gate pattern. However, theeffectiveness of the OPC cannot be checked too carefully because thecircuit is inoperative anymore should any convenience be caused. Thus,in checking the effectiveness, not only the predetermined processconditions, but also process conditions with margins should be takeninto account because the process conditions are variable duringreal-world fabrication process.

If the effectiveness of the OPC is negated in Step SC8 (i.e., if it hasbeen determined that the circuit would not operate properly), then themethod proceeds to Step SC9. In Step SC9, it is determined whether ornot there is any pattern placement that makes the OPC ineffective in thecircuit pattern.

If the answer to the query in Step SC9 is NO, then Steps SC7, SC5, SC6and SC8 are repeatedly performed to remake the cell-level OPC patternsuch that the OPC becomes effective. Alternatively, if the answer to thequery in Step SC9 is YES, then the method proceeds to Step SC4, in whichthe circuit patterns are modified to eliminate the pattern placementthat makes the OPC ineffective. Then, respective steps from SC2 on arerepeatedly performed.

Then, in Step SC10, it is determined whether or not the cell area, orthe total area of circuit patterns, is smaller than its target value. Ifthe cell area is found larger than the target value, then the methodproceeds to Step SC4, in which the circuit patterns are modified toreduce the cell area. Alternatively, if the cell area is found equal toor smaller than the target value, then the method proceeds to Step SC11shown in FIG. 11.

Thereafter, in Step SC11 shown in FIG. 11, the cell-level OPC patterns,which have been prepared on the cell-by-cell basis, are registered at acell library for use to make mask patterns for respective circuitpatterns. The circuit patterns belonging to the second category areregistered at the cell library as they are. As a result, data aboutbasic circuit components, of which an LSI chip pattern will be made, canbe collected.

Next, in Step SC12, data about the circuit patterns needed to fabricatean LSI is extracted from the cell library and LSI chip data is generatedusing the circuit pattern data extracted.

Subsequently, in Step SC13, final process conditions for the realizationof the LSI chip data are determined.

Then, in Step SC14, it is estimated more accurately in accordance withthe final process conditions how much an actual size would deviate froma mask size due to proximity effects. Based on the results of thisestimation, a chip-level OPC pattern is made for portion of a cellbelonging to the second category, e.g., portion in which the actual gatelength of a gate pattern should exactly match a designed one. In thiscase, the OPC pattern can be made by either the rules-based ormodels-based technique.

Next, in Step SC15, it is determined whether or not the dimensions of anactual pattern that will be made from the chip-level OPC pattern, i.e.,critical dimensions (CD), match those of the originally designed circuitpattern. In this case, the CD's are also checked by some simulationtechnique that can reproduce a real circuit precisely. It should benoted, however, that it is not necessary to check the CD for each andevery part of a single circuit pattern. In actuality, the CD's have tobe checked in only a portion of a circuit pattern, where the actualsizes should exactly match the designed sizes. If it has been determinedthat the CD's are not equal to each other, then the method proceeds toStep SC16, in which the chip-level OPC pattern is further modified toeliminate the portions in question from the OPC pattern. Then, StepsSC14 and SC15 are repeatedly performed. It should be noted that if themodels-based technique is applied in Step SC14, then the CD's do nothave to be checked.

Finally, in Step SC17, mask pattern data is generated using the cell-and chip-level OPC patterns. Based on the mask pattern data obtained inthis manner, a mask or reticle is produced and a pattern that will makea resulting circuit fully operative can be transferred onto a resistfilm on a semiconductor substrate, for example, by using the mask orreticle.

Modified Example of Embodiment 2

Hereinafter, a modified example of the second embodiment will bedescribed with reference to the accompanying drawings.

FIGS. 13 and 14 illustrate a processing flow of an LSI mask datagenerating method according to a modified example of the secondembodiment. In the second embodiment, cell-level OPC patterns aredirectly registered at the cell library in Step SC11. In contrast,according to this modified example, the step of making a cell-level OPCpattern includes the sub-steps of: setting the specification of thecell-level OPC pattern to be made; and making the cell-level OPC patternto the specifications by a rules-based technique. In this manner, acircuit pattern and the specification of a cell-level OPC patternassociated with the circuit pattern can be registered at the celllibrary, instead of directly registering the cell-level OPC pattern.

The process steps SD1 through SD10 according to this modified exampleshown in FIG. 13 are different from the process steps SC1 through SC10according to the second embodiment in the following respects.Specifically, Step SC5 of making a cell-level OPC pattern for eachcircuit pattern in FIG. 10 is subdivided into two steps SD5A and SD5B ofsetting specifications of a cell-level OPC pattern to be made and makingthe cell-level OPC pattern, respectively, as shown in FIG. 13.

The process steps SD11 through SD17 according to this modified exampleshown in FIG. 14 are different from the process steps SC11 through SC17according to the second embodiment in the following respects.Specifically, in Step SD11 of forming a cell library, not the OPCpattern itself, but each circuit pattern and specifications of acell-level OPC pattern associated with the circuit pattern areregistered in combination at the cell library.

In addition, a big difference between this modified example and thesecond embodiment is that in Step SD14, a cell-level OPC pattern is madeto the specifications of the cell-level OPC pattern, and at the sametime, a chip-level OPC pattern is also made to specifications of thechip-level OPC pattern by either the rules-based or models-basedtechnique.

According to this modified example, there is no need to process thecell- and chip-level OPC patterns, each being made up of a huge volumeof complicated pattern data, until just before the mask data isgenerated. Thus, that huge volume of data can be processed in a singleprocess step.

Also, a cell to be registered at the cell library needs to represent notonly mask data for use in producing a mask but also a circuitconfiguration. Thus, a pattern to be registered is preferably a circuitpattern representing an expected actual pattern, not the OPC pattern.Furthermore, even when a circuit pattern once registered should bechanged, the circuit pattern registered would be more advantageous thanan OPC pattern registered.

As described above, according to the second embodiment and its modifiedexample, circuit patterns are classified into the first categorystrongly affecting a cell area and the second category that does notaffect a cell area so much. Thus, a cell-level OPC pattern belonging tothe first category can be formed while a cell is being designed. That isto say, a circuit pattern can be designed to make the OPC fullyeffective and to reduce the cell area as much as possible, or a patternplacement that makes OPC ineffective can be eliminated while eachcircuit pattern is being designed. Accordingly, once a target cell areahas been attained, there can be neither hardly-realizable circuitpattern nor circuit pattern with unnecessarily large margin. As aresult, an expected yield of normally operating LSI's can be increasedwhile minimizing a cell area to its target value.

Embodiment 3

Hereinafter, a third embodiment of the present invention will bedescribed with reference to the accompanying drawings.

According to the third embodiment, an OPC pattern, which is associatedwith circuit patterns defined by multiple layers, is made in an earlierprocess step of designing a cell. For example, the OPC pattern for atransistor, which includes a first layer with an active layer patternand a second layer with a gate pattern overlapping with the active layerpattern, is made earlier than the second embodiment. On the other hand,an OPC pattern, which is associated with circuit patterns defined by asingle layer such as a gate pattern, is made in a later process step ofprocessing mask data.

In general, a circuit (or a cell) includes multiple components. Circuitpatterns may be laid out in various manners through the placement ofthese components to form many different types of circuits. Among thesecircuits, one strongly affecting a cell area is defined by patternscovering multiple layers in most cases, and is rarely defined bysingle-layer patterns. In an OPC pattern defined by multiple layers, theplacement of circuit components is modifiable on a layer-by-layer basis.Thus, if that OPC pattern defined by multiple layers is made as early asat the cell design stage, then the circuit components can be placed tofurther reduce the cell area.

FIGS. 15 and 16 illustrate a processing flow of an LSI mask datagenerating method according to the third embodiment.

First, in Step SE1 shown in FIG. 15, design rules, basic processconditions and OPC pattern placement rules are defined as in Step SC1 ofthe second embodiment. In addition, an OPC pattern that is defined bymultiple layers and will play a key role in determining a cell area(hereinafter, such OPC pattern will be called an “interlayer OPCpattern”) is classified as a first category. On the other hand, an OPCpattern defined by a single layer (hereinafter, such an OPC pattern willbe called an “intralayer OPC pattern”) is classified as a secondcategory.

Next, in Steps SE2, SE3 and SE4, respective circuit patterns are formedand it is determined whether or not each of these circuit patterns meetsthe design rules. Then, in Step SE5, the interlayer OPC patterns aremade for each cell.

Hereinafter, the interlayer OPC patterns will be described withreference to the accompanying drawings.

FIG. 17( a) shows a planar layout for a transistor circuit illustratingcircuit patterns according to this embodiment, while FIG. 17( b) shows aplanar layout for a transistor circuit illustrating interlayer OPCpatterns corresponding to the circuit patterns shown in FIG. 17( a). Asshown in FIG. 17( a), first and second circuit patterns 31A and 32A areplaced in this transistor circuit. Each of the first and second circuitpatterns 31A and 32A includes a rectangular active layer pattern 31 a,32 a and a gate layer pattern 31 b, 32 b overlapping the center of thelonger sides of the active layer pattern 31 a, 32 a. A longer side ofone active layer pattern 31 a is spaced apart from the adjacent longerside of the other active layer pattern 32 a by about 0.3 μm. Also, thegate layer patterns 31 b and 32 b are placed such that their adjacentedges do not overlap with each other, but are spaced to face each other.

In this manner, the first circuit pattern 31A has a region in which thegate layer pattern 31 b overlaps with the active layer pattern 31 a.Thus, if the active layer pattern 31 a is formed in a semiconductorsubstrate, then a channel region is defined in the overlapping region,thereby making the circuit pattern 31A operate as a transistor.Accordingly, there is a placement rule to be followed by the activelayer pattern 31 a and the gate layer pattern 31 b. In other words, ifone of the active layer and gate layer patterns 31 a and 31 b haschanged its location, then both of these patterns 31 a and 31 b aremutually affected. The same statement is equally applicable to thesecond circuit pattern 32A.

First and second OPC patterns 31B and 32B shown in FIG. 17( b)correspond to the first and second circuit patterns 31A and 32A shown inFIG. 17( a), respectively. As shown in FIG. 17( b), hammerhead patternsin different shapes are added to both edges of each gate layer pattern31 b, 32 b. More specifically, the hammerhead patterns added to theclosely placed edges of the gate layer patterns 31 b and 32 b aresmaller in area than those added to the other edges of the gate layerpatterns 31 b and 32 b such that the active layer patterns 31 a and 32 ain the OPC patterns are spaced apart by 0.2 μm, which is smaller thanthat of the circuit patterns.

Although not shown, as for a contact pattern formed to interconnecttogether lines included in mutually different layers, line OPC patternsmade from line patterns and a contact OPC pattern made from the contactpattern are defined by multiple layers.

Subsequently, in Steps SE6, SE8 and SE10 shown in FIG. 15, theinterlayer OPC pattern obtained is checked as to whether the patternmeets the OPC pattern placement rules, whether the pattern makes the OPCeffective, and whether the cell area is the predetermined value or less,respectively. These items may be checked by the methods exemplified inthe second embodiment. If the check result is no-go, then the interlayerOPC pattern should be modified in Step SE7 or the circuit pattern ismodified layer by layer and the circuit pattern elements are placedagain in Step SE4 to make the OPC effective.

Thereafter, in Step SE11 shown in FIG. 16, the interlayer OPC patterns,which have been prepared on the cell-by-cell basis, are registered at acell library for use to make mask patterns for respective circuitpatterns. The circuit patterns belonging to the second category areregistered at the cell library as they are. As a result, data aboutbasic circuit components, of which an LSI chip pattern will be made, canbe collected.

Next, in Step SE12, data about the circuit patterns needed to fabricatean LSI is extracted from the cell library and LSI chip data is generatedusing the circuit pattern data extracted. Subsequently, in Step SE13,final process conditions for the realization of the LSI chip data aredetermined.

Then, in Step SE14, it is estimated more accurately in accordance withthe final process conditions how much an actual size would deviate froma mask size due to proximity effects. Based on the results of thisestimation, an intralayer OPC pattern belonging to the second categoryis made. In this case, the OPC pattern may be made by either therules-based or models-based technique.

Next, in Step SE15, it is determined whether or not the dimensions of anactual pattern that will be made from the intralayer OPC pattern, i.e.,critical dimensions (CD), match those of the originally designed circuitpattern. In this case, the CD's are also checked by some simulationtechnique that can reproduce a real circuit precisely. It should benoted, however, that it is not necessary to check the CD for each andevery part of a single circuit pattern. In actuality, the CD's have tobe checked in only a portion of a circuit pattern, where the actual sizeshould exactly match the designed size. If it has been determined thatthe CD's are not equal to each other, then the method proceeds to StepSE16, in which the intralayer OPC pattern is further modified toeliminate the portions in question from the OPC pattern. Then, StepsSE14 and SE15 are repeatedly performed. It should be noted that if themodels-based technique is applied in Step SE14, then the CD's do nothave to be checked.

Finally, in Step SE17, mask pattern data is generated using theinterlayer and intralayer OPC patterns. Based on the mask pattern dataobtained in this manner, a mask or reticle is produced and a patternthat will make a resulting circuit fully operative can be transferredonto a resist film on a semiconductor substrate, for example, by usingthe mask or reticle.

In the foregoing embodiment, the intralayer OPC pattern belonging to thesecond category is made in Step SE14 after LSI chip data has beendefined in Step SE12. However, the cell-level OPC patterns classified asthe first category according to the second embodiment are also includedin the intralayer OPC patterns. Accordingly, such a circuit, for whichthose cell-level OPC patterns are made, may be designed in Step SE2.

Modified Example of Embodiment 3

Hereinafter, a modified example of the third embodiment will bedescribed with reference to the accompanying drawings.

FIGS. 18 and 19 illustrate a processing flow of an LSI mask datagenerating method according to a modified example of the thirdembodiment. In the third embodiment, interlayer OPC patterns aredirectly registered at the cell library in Step SE11. In contrast,according to this modified example, the step of making an interlayer OPCpattern by the rules-based technique includes the sub-steps of: settingthe specification of the interlayer OPC pattern to be made; and makingthe interlayer OPC pattern to the specifications. In this manner, acircuit pattern and the specification of an interlayer OPC patternassociated with the circuit pattern can be registered at the celllibrary, instead of directly registering the interlayer OPC pattern.

The process steps SF1 through SF10 according to this modified exampleshown in FIG. 18 are different from the process steps SE1 through SE10according to the third embodiment shown in FIG. 15 in the followingrespects. Specifically, Step SE5 of making an interlayer OPC pattern inFIG. 15 is subdivided into two steps SF5A and SF5B of settingspecifications of an interlayer OPC pattern to be made for each circuit(or cell) and making the interlayer OPC pattern, respectively, as shownin FIG. 18.

The process steps SF11 through SF17 according to this modified exampleshown in FIG. 19 are different from the process steps SE11 through SE17according to the third embodiment in the following respects.Specifically, in Step SF11 of forming a cell library, not the OPCpattern itself, but each circuit pattern and specifications of aninterlayer OPC pattern associated with the circuit pattern areregistered in combination at the cell library.

In addition, a big difference between this modified example and thethird embodiment is that in Step SF14, an interlayer OPC pattern is madeto the specifications of the interlayer OPC pattern, and at the sametime, an intralayer OPC pattern is also made to specifications of theintralayer OPC pattern by either the rules-based or models-basedtechnique.

According to this modified example, there is no need to process theinterlayer and intralayer OPC patterns, each being made up of a hugevolume of complicated pattern data, until just before the mask data isgenerated. Thus, that large volume of data can be processed in a singleprocess step.

Also, a cell to be registered at the cell library needs to represent notonly mask data for use in producing a mask but also a circuitconfiguration. Thus, a pattern to be registered is preferably a circuitpattern representing an expected actual pattern, not the OPC pattern.Furthermore, even when a circuit pattern once registered should bechanged, the circuit pattern registered would be more advantageous thanan OPC pattern registered.

As described above, according to the third embodiment and its modifiedexample, circuit patterns are classified into the first categorystrongly affecting a cell area and the second category that does notaffect a cell area so much. Thus, an interlayer OPC pattern belonging tothe first category can be formed while a cell is being designed. That isto say, a circuit pattern can be designed to make the OPC fullyeffective and to reduce a cell area as much as possible, or a patternplacement that makes OPC ineffective can be eliminated while eachcircuit pattern is being designed. Accordingly, once a target cell areahas been attained, there can be neither hardly-realizable circuitpattern nor circuit pattern with unnecessarily large margin. As aresult, an expected yield of normally operating LSI's can be increasedwhile minimizing a cell area to its target value.

In Steps SE4 and SF4, a tool called “compactor” may be used for placingcircuit pattern elements again. In such a case, there is no need torepeatedly perform check and modification as is done in the thirdembodiment. Furthermore, if the compactor is made to obey a rule, whichshould be followed to perform OPC fully effectively using the interlayerOPC pattern, during the re-placement, then cell patterns can also besynthesized automatically.

1-46. (canceled)
 47. A method for forming an LSI pattern, the LSIpattern including a plurality of circuit patterns, the method comprisingthe steps of: a) designing the circuit patterns; b) making an initialplacement for the circuit patterns designed; c) performing opticalproximity corrections on at least two of the circuit patterns that havebeen initially placed to be adjacent to or cross each other, therebyforming optical proximity corrected patterns out of the adjacent orcrossing circuit patterns; d) evaluating effectiveness of the proximitycorrections under predetermined process conditions; if the effectivenessof the corrections is negated, e) changing a design rule defining thecircuit patterns to make the corrections effective; f) making are-placement for the initially placed circuit patterns in accordancewith the design rule changed; g) producing a mask using the correctedpatterns; and h) defining the circuit patterns on a semiconductorsubstrate under the predetermined process conditions by using the maskproduced; wherein the step h) includes the step of forming transferredpatterns on a resist film on the semiconductor substrate using the maskproduced.
 48. The method of claim 47, further comprising the steps of:estimating process yield expectedly attainable when the produced mask isused under the predetermined process conditions after the step g) hasbeen performed; and if the estimate is short of a target value,modifying the predetermined process conditions to make the estimatereach the target value and repeatedly performing the steps a) throughg).
 49. The method of claim 47, wherein the step of forming thecorrected patterns includes the step of setting specifications of thecorrected patterns to be made, and wherein the step of evaluatingincludes the step of changing the specifications to make the correctionseffective if the effectiveness of the corrections is negated.
 50. Themethod of claim 47, wherein the step of making the re-placement includesthe step of forming multiple re-placement patterns and selecting one ofthe re-placement patterns that minimizes a total circuit area.
 51. Themethod of claim 47, further comprising the step of defining the designrule such that the circuit patterns can be laid out to make thecorrections effective, wherein the step of making the initial placementor the step of making the re-placement includes the step of placing thecircuit patterns in accordance with the design rule.
 52. The method ofclaim 47, wherein the step of defining the design rule includes the stepof defining multiple design rules and selecting one of the design rulesthat minimizes a total circuit area.
 53. The method of claim 47, furthercomprising the steps of: setting specifications of the correctedpatterns to be made; defining a rule of placing the corrected patternssuch that the proximity corrections can be performed effectively usingthe corrected patterns; and forming the corrected patterns according tothe specifications of the corrected patterns to be made and on the ruleof placing the corrected patterns, thereby defining the design rule. 54.The method of claim 47, further comprising the steps of: evaluatingeffectiveness of the proximity corrections for the circuit patterns thathave been placed in accordance with the design rule; and modifying thespecifications of the corrected patterns to be made or the rule ofplacing the corrected patterns to make the corrections effective if theeffectiveness of the corrections is negated.
 55. The method of claim 47,wherein the step of evaluating includes the step of determining whetheror not an expected actual size falls within a predetermined range byperforming a process simulation including at least one of lithographyand etching process steps.
 56. The method of claim 55, wherein in theprocess simulation, it is determined whether or not an expected size ofa transistor in a gate longitudinal direction falls within thepredetermined range.